The effects of addition of a filler to fluororesins are greatly influenced by the morphology, particle diameter and chemical properties of filler particles. Generally, an improvement in filler dispersibility produces beneficial effects on mechanical strength characteristics, for example tensile strength, rigidity, and hardness. Further, an improvement in filler dispersibility is highly effective in producing improvements in thermal conductivity, chemical resistance, and workability, for instance.
For improving the dispersibility of a filler having low affinity for fluororesins, the filler is surface-modified, for example, by treatment with a silane coupling agent or by fluorination treatment to provide it with affinity for fluororesins.
However, such filler surface modification requires the use of a complicated equipment and, in addition, unfavorably results in an increase in filler production cost. Therefore, a method of uniformly dispersing, in fluororesins, a filler having low affinity for fluororesins without surface modification is demanded.
For example, conductive wires to be used for transmitting electronic signals are required to be isolated so that accurate transmission may be secured. Such isolation is realized, for example, by melting a resin and extruding it around a conductive wire to form a covering material. Judiciously used as such resin are fluororesins because of their being low in permittivity, lightweight and excellent in heat resistance, flame resistance and smokelessness, among others.
For attaining improvements in electrical characteristics, the covering material for conductive wires is desired to have a reduced permittivity. For reducing permittivity, it is effective to mold the covering material in a foamed moldings. Foamed moldings made from a resin are generally produced by foam molding, namely molding a molten resin while allowing a gas to be present therein. For obtaining foamed moldings uniform in shape and characteristics, it is desirable that the foam cells in the foamed moldings are fine and uniformly distributed.
For the purpose of rendering foam cells fine and uniformly distributed, there is a method known which comprises causing a nucleating agent for foaming to be present in resins so that the agent may form the origins for foam cell formation in the step of foam molding. Preferably used as the nucleating agent for foaming to be added to such resins as fluororesins is boron nitride (BN) since this is thermally stable, chemically inert and low in toxicity and has favorable electrical properties, for example it causes only slight changes in permittivity due to addition thereof.
As regards the technology of obtaining foamed moldings by incorporating BN as a nucleating agent for foaming in resins such as fluororesins, there are the following disclosures about BN, among others:
Japanese Kokai Publication Hei-8-12796 discloses that the combination of BN and zeolite is optimal among the combinations of ceramics or starting materials thereof with zeolite as nucleating agents for foaming.
Japanese Kokai Publication Sho-63-24503 and Japanese Kokai Publication Sho-63-110508 disclose methods which use BN as a nucleating agent for foaming.
Japanese Kokai Publication Sho-59-11340 discloses a method comprising adding BN having a surface area of 5 to 10 m2/g as a nucleating agent for foaming.
Japanese Kokoku Publication Hei-6-89166, U.S. Pat. No. 4,877,815 and U.S. Pat. No. 5,023,279 disclose the combined use of BN having a surface area of 8.6 m2/g with a sulfonic acid, a phosphonic acid or a salt thereof.
Japanese Kokoku Publication Hei-07-121999 and Japanese Kokai Publication (U.S. Pat. No. 4,764,538) disclose the combined use of BN having a surface area of about 8 m2/g with a thermally stable inorganic salt composed of a metal cation and a polyatomic anion and satisfying a certain relation, for example sodium tetraborate.
U.S. Pat. No. 3,072,583 discloses that the use of BN having an average particle diameter smaller than 10 μm is preferred in forming foamed fluororesin moldings and that the resin and BN is ground in an appropriate mill.
Japanese Kokai Publication Hei-10-195216 discloses a method which comprises mixing up a resin powder capable of foaming by decarboxylation, BN and a metal salt to give a blend, granulating the blend with a compactor and then with a hammer mill or a like grinder to give a powder, or pelletizing the blend with a pellet mill, and the pellets to an extruder and, according to the disclosure, the use of a Henschel mixer (product of Purnell International) is preferred in blending.
Japanese Kokai Publication Hei-10-45931 discloses a foaming method comprising using a processable fluorine-containing polymeric substance and BN in combination with a sulfonic acid or phosphonic acid or an acid salt thereof and/or a polyatomic-anion-containing inorganic salt, said BN having a form of crystals grown to a final size with an average particle size of not larger than 12 μm. It is described that this BN is prepared not by mechanical grinding of crushing individual crystals but by deagglomeration in case of occurrence of agglomeration of individual grown crystallites.
However, in any of these methods of obtaining foamed moldings by incorporating a nucleating agent in resins, there is no attention paid to the particle size distribution of the nucleating agent material or, further, to the particle size distribution of the nucleating agent in resin compositions.
These prior art technologies are not fully satisfactory in the existing circumstances in which a covering material further improved in electrical characteristics is demanded as a result of the increase in the volume of information to be transmitted at a time as resulting from the increase in the speed of communication owing to the recent advances in information-related technologies.